Orthogonal Frequency Division Multiple Access (OFDMA) modulation schemes have been proposed for downlink transmissions over an air interface in next generation communication systems such as 3GPP (Third Generation Partnership Project) E-UTRA (Evolved UMTS Terrestrial Radio Access) and 3GPP2 Phase 2 communication systems. In an OFDMA communication system, a frequency channel, or bandwidth, is split into multiple contiguous frequency sub-bands (also known as resource blocks (RBs)). In turn, each sub-band comprises multiple, for example, 12, contiguous frequency sub-carriers that are orthogonal to each other. Under the 3GPP E-UTRA standards, a Node B then assigns resource blocks to users equipment (UEs) on a sub-frame basis, wherein a sub-frame has a duration of one millisecond (ms). Within one sub-frame, distributed (for frequency diversity) and localized (resource block-based) transmission modes are multiplexed in an FDM manner.
In a 3GPP E-UTRA communication system, a Physical Resource Block (PRB) consists of a set of contiguous sub-carriers (for example, 12) over a number of OFDM symbols (for example, 12). A PRB contains data from only a localized user or distributed users but not both. However, UEs are not assigned a PRB. Instead, UEs are assigned a Virtual Resource Block (VRB), which is a logical resource block that is associated with a same number of sub-carriers, again, 12 for example, as a PRB. The VRB is then mapped to one or more PRBs. Two separate schemes have been proposed for such mappings. One scheme, known as a Localized VRB (LVRB), maps a VRB into a single PRB, that is, maps the 12 sub-carriers of a VRB to the 12 sub-carriers of a corresponding PRB. Localized mapping is used for Frequency Selective Scheduling (FSS), wherein transmission errors are minimized by scheduling a user equipment (UE) for a PRB only where the UE is known to have a good downlink channel. Accordingly, FSS requires narrowband channel feedback from the UE. A narrowband channel feedback is specific to each PRB while wideband channel feedback is the channel quality over the whole bandwidth. A second scheme, known as a Distributed VRB (DVRB), maps a VRB into multiple PRBs, that is, the 12 sub-carriers of a VRB are mapped to one or more sub-carriers of each of multiple PRBs. Distributed mapping is used for Frequency Diverse Scheduling (FDS), wherein a VRB is distributed among multiple PRBs without channel feedback or only wideband channel feedback and thus reduces the potential for transmission errors by use of frequency diversity.
An LVRB is most efficient for high data rate users and data transfer applications, such as File Transfer Protocol (FTP), where low error rates are required. However, an LVRB requires narrowband channel quality feedback, typically for each LVRB that may be scheduled, which consumes uplink system capacity. By contrast, a DVRB may be more efficient for services such as VoIP, as it may use distributed transmission and require very little or no feedback.
Therefore, a need exists for a method and apparatus that can maximize bandwidth utilization and minimize transmission errors by scheduling both DVRBs and LVRBs in a same frequency channel, such that the benefits of frequency selective scheduling is obtained while minimizing the uplink feedback overhead.
One of ordinary skill in the art will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help improve understanding of various embodiments of the present invention. Also, common and well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention.